This application is related to Japanese application No. Hei 11(1999)-284349 filed on Oct. 5, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a lithium secondary cell having solid electrolytes.
2. Description of the Related Art
A secondary cell has been widely used as a power supply for portable appliances in view of economics. The secondary cell includes various types. At present, a nickel-cadmium cell is most common, and a nickel-hydrogen cell has been lately widespread. Further, a lithium secondary cell which is superior to the nickel-cadmium cell or the nickel-hydrogen cell in terms of output voltage and energy density is becoming a leading secondary cell.
In this lithium secondary cell, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) or a solid solution thereof (Li(Co1xe2x88x92xNix)O2 in which 0 less than x less than 1) or lithium manganese oxide (LiMn2O4) having a spinel structure is used in a positive electrode, a negative electrode material made of carbon, such as graphite is used in a negative electrode, and an electrolyte in which a solvent is a liquid organic compound and a solute is a lithium compound is used.
In this lithium secondary cell, some problems have been pointed out. One of them is caused by a combination of an electrolyte and carbon, which is a negative electrode material.
Graphite as a negative electrode material has a potential, to. metallic lithium, of 0.1 to 0.2 V on average which is close to that of metallic lithium, and a capacity which is as great as 375 mAh/g. Thus, it is preferable as a negative electrode material of the lithium secondary cell.
However, when graphite is used as the negative electrode material and propylene carbonate as a solvent, propylene carbonate is reduced in a reaction of lithium insertion to cause decomposition. Thus, it cannot be used in a cell.
In order to solve this problem, the following two methods have been generally used at present.
A first method is one using an electrolyte in which a solvent is ethylene carbonate. When ethylene carbonate is used as a solvent, the decomposition reaction of the electrolyte on carbon is inhibited, and lithium can be inserted into or deserted from carbon. However, since ethylene carbonate has a melting point of 39.0xc2x0 C., solidification occurs at room temperature with ethylene carbonate when used alone. Thus, it cannot be used in a cell. Accordingly, the melting point can be decreased by mixing ethylene carbonate with a different organic solvent. Nevertheless, a conductivity is extremely decreased at a temperature of approximately 0xc2x0 C., and it cannot be used in a cell.
Thus, when ethylene carbonate is used as a solvent, low-temperature characteristics are worsened.
A second method is one in which a carbonaceous material having a relatively low degree of graphitization is used in a negative electrode. In the carbonaceous material, propylene carbonate is used as an electrolyte, whereby a reaction of lithium insertion or desertion proceeds without a decomposition reaction.
Nevertheless, the carbonaceous material having a relatively low crystallinity has a high potential to lithium compared with graphite having a high crystallinity. Accordingly, when a lithium secondary battery is constructed from a combination of such a carbonaceous material with a positive electrode material, there is a defect that its output potential is lower than that of a lithium secondary battery using graphite having a high crystallinity.
That is, a lithium secondary cell currently on the market is classified into a cell using ethylene carbonate and a graphite material and having poor low-temperature characteristics and a cell using propylene carbonate and a carbonaceous material and having a poor output potential.
Meanwhile, a solid electrolyte obtained by solidifying or gelling an organic electrolyte with a polymer is also available. However, since the solid electrolyte contains ethylene carbonate or propylene carbonate in the electrolyte, it still involves the problems that the decomposition on carbon occurs, the low-temperature characteristics are poor and the output potential is decreased.
Japanese Unexamined Patent Publication No. Hei 8(1996)-222235 discloses that the decomposition of a solid electrolyte is inhibited by using a solid electrolyte having a noble potential window in a solid electrolyte in contact with a positive electrode and a solid electrolyte having a base potential window in a solid electrolyte in contact with a negative electrode, whereby a cell having a low inner resistance and a great discharge capacity can be formed. This document however concerns with a technique for inhibiting the decomposition of the electrolyte, and low-temperature characteristics and load characteristics still remain unresolved.
Under these circumstances, an object of the invention is to provide a lithium secondary cell having a high output potential and excellent in low-temperature characteristics and negative electrode characteristics. The present invention provides a lithium secondary battery comprising a positive electrode containing a first solid electrolyte; a negative electrode containing a second solid electrolyte; and a layer of a third solid electrolyte between the positive and negative electrodes.
In order to overcome the problems, the present inventors have conducted various investigations and have consequently found that the problems can be solved by using solid electrolytes containing different electrolytes in a positive electrode and a negative electrode. That is, various electrode materials and organic solvent electrolytes have been studied, and it have consequently been found that a lithium secondary cell having a high output potential and excellent in low-temperature characteristics and load characteristics can be provided by incorporating solid electrolytes having different compositions in a positive electrode and a negative electrode and interposing a solid electrolyte layer having a third composition between the two electrodes.